The ever increasing power of fiber lasers creates a demand for high power handling components. These high-power fiber lasers are based on double-clad optical fiber having an inner rare-earth doped silica core and an inner cladding. This configuration is necessary for pump light coupling and subsequent pump absorption along the length of the double-clad optical fiber. The double-clad optical fiber further comprises an inner cladding with high numerical aperture to couple low brightness pump light. This is achieved by surrounding the inner cladding with an external polymer outer cladding having a refractive index lower than the refractive index of silica. The double-clad optical fiber further comprises an external polymer overcoat having a refractive index higher than the refractive index of silica to provide mechanical integrity. However, currently used polymer overcoats and outer claddings have a relatively low operating temperature range. Because of the limited operating temperature range of known polymer overcoats and outer claddings, high power fiber lasers based on double-clad optical fiber require proper stray light and temperature management, to avoid failure caused by thermal decay of the polymer overcoat.
Frequently, in double-clad optical fiber lasers and amplifiers, undesirable light (such as unabsorbed pump light, light that escapes the double-clad fiber core, or feedback light from material processing) propagates in the inner cladding of the double-clad optical fiber core. This undesirable light, if not stripped out of the double-clad optical fiber, may reduce the output beam quality of the high-power laser or create catastrophic failure of the laser system through thermal decay. The unwanted light is usually stripped by introducing a cladding mode stripper which removes the waveguiding effect of the inner cladding. An example of prior cladding mode stripper is the typical polymer based cladding mode stripper which replaces the low refractive index polymer outer cladding by a material having a refractive index higher than the refractive index of silica, so that the undesirable light is fully extracted from the inner cladding after a certain length. However, any refractive index value higher than the refractive index of the outer cladding will extract light from the inner cladding by reducing the numerical aperture of the inner cladding waveguide. Techniques aimed at improving power handling capacity of conventional cladding mode strippers are well known in the art. The reliability of cladding mode strippers being related to a peak temperature in the polymer overcoat of the double-clad optical fiber, known techniques attempt to either distribute the heat along the double-clad optical fiber by controlling the rate of light stripping, or reduce the peak temperature by packaging methods.
Wetter et al., in a scientific paper entitled “High power cladding light strippers” (Photonics West 2008, Fiber Lasers V.' Technology, Systems, and Applications, Proc. of SPIE Vol. 6873, 687327), discloses a gradual cladding mode stripper in which the stripping material has a varying refractive index along the double-clad active optical fiber.
Anderegg et al. (in U.S. Pat. No. 7,349,596) discloses a cladding mode stripper applying a stripping material as the outer cladding, said stripping material having a refractive index with negative temperature dependence. By increasing the temperature of the stripping material, the light stripping rate is lowered, and thus the peak temperature in the cladding mode stripper saturates.
Kliner et al. (in U.S. Pat. No. 8,027,555) discloses a cladding mode stripper where the extracted light passes through a block of transparent material to be absorbed.
Finally, Freier et al. (in U.S. Pat. No. 6,301,418) discloses a waveguide light diffuser with non-uniform cladding roughness or indentations.
These different techniques for stripping the undesirable light from a double-clad optical fiber, as disclosed in the aforementioned references, are not easily achieved in practice. Varying the refractive index along the double-clad optical fiber requires a very good spatial resolution of refractive index change. Also, more than one type of polymer with specific low refractive index may be needed, to cover a range of refractive indexes to perform cladding mode stripping. There are fewer choices of materials having a low refractive index with the desired optical and mechanical properties, than materials having a high refractive index. Generally, the operating temperature range of materials having a low refractive index is lower than the operating temperature range of materials having a high refractive index. The same issues also occur for materials having a refractive index with negative temperature dependence. Furthermore, having surface roughness or indentations on a core and inner cladding made of silica reduces its mechanical strength.
Accordingly, there is a need for providing an easier control of undesirable light extraction, to even out the temperature profile within the cladding mode stripper.